This invention relates to an improved metal treatment process, and more particularly, to a new and improved method for inhibiting the corrosion of metal articles. The invention relates particularly to the treatment of small metal parts and subassemblies to improve the corrosion resistance of such parts. The process can include treating single parts and bulk barrel processing of parts.
It is well known in the metalfinishing art that metal surfaces such as aluminum, ferrous, and zinc surfaces may be provided with an inorganic phosphate coating by contacting them with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as an excellent base for the later application of siccative organic coating compositions such as paint, lacquer, varnish, primers, synthetic resins, enamel, and the like.
The inorganic phosphate coatings generally are formed on a metal surface by means of aqueous solutions which contain phosphate ion and, optionally, certain auxiliary ions including metallic ions such as sodium, manganese, zinc, cadmium, copper, lead, and antimony ions. These aqueous solutions also may contain nonmetallic ions such as ammonium, chloride, bromide, fluoride, nitrate, sulfate, and borate ions. These auxiliary ions influence the reaction with the metal surface, modify the character of the phosphate coating, and adapt it for a wide variety of applications. Other auxiliary agents such as oxidizing agents, coloring agents, and metal cleaning agents also may be incorporated in the phosphating solution.
As mentioned above, the inorganic phosphate coatings have been found to provide an excellent base for the application of siccative organic coatings such as paints or lacquers. The provision of such phosphate coatings has been found to improve both the adhesion of the paint or lacquer film to the metal surface and the corrosion resistance of the painted metal.
Solvent-base siccative organic coating compositions have been applied to metal surfaces such as by spraying, dipping, rolling, centrifuged dip-spinning, etc. More recently, with the development of various water-soluble resin base paints and lacquers, a great deal of effort has been made toward the application of such coating systems by electrophoresis. The electrophoretic application of paint and lacquer involves the phenomena of electro-osmosis and electrolysis, as well as electrophoresis. In this method, an electric current is passed through the paint or lacquer solution while the article to be painted is made an electrode, usually the anode, in the paint or lacquer.
The electrodeposition of water-based coatings although widely used to process sheet metal parts, is not currently employed on subassemblies and barrel processing equipment for stamped parts such as nuts, bolts and fasteners. Electrodeposition for these parts has many advantages over other methods of coating. The process deposits a film of uniform thickness on essentially any conductive surface, even those which have sharp points and edges. The electrocoated film when applied is relatively water-free, and, thus, will not run or drip when taken out of the bath. Because little or no organic solvents are used in the coating system, the process does not require special precautions or equipment for handling any harmful solvent vapors.
Procedures for improving the rust resistance of metal articles by application of a film of paint over a phosphated surface have been described in a number of patents such as U.S. Pat. Nos. 3,620,949; 3,864,230; 3,454,483; 4,007,102 and Re. 27,896. As noted in U.S. Pat. No. Re. 27,896, the electrophoretic application of paint and lacquer over a phosphated metal surface is not a complete solution to the rust problem. It has been found that when paint is electrodeposited on phosphate coated ferrous metal surfaces, the resulting paint films have often been found to contain numerous small depressions or pin holes. Such films generally provide only a slight corrosion protective action, probably due to the presence of a lower film thickness in the depressions. In an attempt to overcome this problem, paints and lacquers have been used containing synthetic resin components which form films which during a subsequent baking will soften so that the surface blends smoothly and the depressions and pin holes are filled with resin. One difficulty with this technique, however, is that there often is a withdrawal of the paint film from the edges of the workpiece being treated so that these portions are then subjected to additional corrosion attacks. In U.S. Pat. No. Re. 27,896, a solution to this problem is suggested which involves the incorporation of cupric ions into the zinc phosphate coating applied to the article prior to painting. In U.S. Pat. No. 3,454,483, an improvement in the corrosion resistance of electrophoretically painted metal surfaces is suggested when the phosphate coating used as a primer for the paint contains fluoride ions.